The present invention is directed to overcoming the problem of printing on glossy or semi-glossy, clay-coated papers or the like with aqueous inkjet inks. Currently available glossy or semi-glossy coated papers of this kind have been engineered over the years to be compatible with conventional, analog printing technologies, such as offset lithography. The printing inks used in offset printing processes are typically very high solids, and the solvents are often non-aqueous. As a consequence, clay-based coatings that are currently used on glossy or semi-glossy printing papers, such as those used for magazines and mail order catalogs, have been intentionally designed to be resistant to the absorption of water. In fact, when these papers are characterized by standard tests as to their porosity and/or permeability, they have been found to be essentially impermeable. When such clay-coated papers are printed with inkjet inks that comprise as much as 90-95% water as the carrier solvent, the inks have a tendency to sit on the surface of the clay coating, rather than penetrate into the coating and/or underlying paper substrate.
Because the inks must dry primarily by evaporation of the water without any significant penetration or absorption of the water into the coating or paper, a number of problems are encountered. One such problem is that the individual ink droplets slowly spread laterally across the surface of the coating, eventually touching and coalescing with adjacent ink droplets. This gives rise to a visual image quality artifact known as “coalescence” or “puddling.” Another problem encountered when inks dry too slowly is that when two different color inks are printed next to each other, such as when black text is highlighted or surrounded by yellow ink, the two colors tend to bleed into one another, resulting in a defect known as “intercolor bleed.” Yet another problem is that when printing at high speed, either in a sheet fed printing process, or in a roll-to-roll printing process, the printed image is not dried sufficiently before the printed image comes in contact with an unprinted surface, and ink is transferred from the printed area to the unprinted surface, resulting in “ink retransfer.”
Such problems have been solved in the prior art by the use of ink-receiving layers that are porous and/or permeable to the ink. However, such coated papers are generally not suitable for high-speed inkjet printing applications for a number of reasons. In general, the glossy or semi-glossy, coated papers suitable for slower, desktop consumer inkjet printing applications, such as digital photography, are too expensive for high-speed inkjet commercial printing applications, such as magazines, brochures, catalogs, and the like. This is because such coated papers require either expensive materials, such as fumed oxides of silica or alumina, to produce a glossy surface or very thick coatings to adequately absorb the relatively heavy ink coverage required to print high quality photographs. Such coated papers may also employ cationic additives, which result in coating formulations that are incompatible with the fluid delivery systems employed by low-cost, high-speed coating technologies used for offset printing grades.
Multivalent metal salts are known to improve the print density and uniformity of images formed on plain papers from inkjet printers. For example, Cousin, et al., in U.S. Pat. No. 4,554,181, disclose the combination of a water-soluble salt of a polyvalent metal ion and a cationic polymer for improving the print density of images printed by inkjet printers employing anionic dye-based inks. Varnell, in U.S. Pat. No. 6,207,258, discloses the use of water-soluble salts of multivalent metal ions to improve the print density and uniformity of images formed on plain papers from inkjet printers employing pigment colorants in the ink set.
Plain paper is not glossy, and traditional glossy papers for lithographic and offset printing have overcoated paper with inorganic particles such as calcium carbonate, kaolin clay, and titanium dioxide to improve smoothness and gloss. However, these inorganic pigments have a net anionic charge, and the addition of multivalent cations to a coating solution containing these anionic pigment particles will lead to agglomeration of the particles and loss of coating gloss. The coating pigments could be made compatible with the metal salts by dispersing them with a cationic dispersant (such as p-DADMAC or poly(dimethylamine)-co-epichlorohydrin), but the resulting cationic coating solution is undesired by most paper manufacturers and coaters due to the potential for contamination and interaction with the anionic coating solutions normally present in these manufacturing facilities. A changeover procedure for thorough cleaning between coating events would be too time-consuming and costly to allow a coating composition containing a cationic component to be used.
Japanese patent application publication JP 2002-264485 discloses an inkjet recording paper with one ink-receiving layer containing a white pigment and a binder coated on a support that contains a water-soluble salt of a polyvalent metal ion. However, the coating composition for the ink-receiving layer also contains a cationic resin, which is not compatible with anionic coating compositions that may be employed on the same manufacturing equipment. U.S. Pat. No. 6,350,507 to Iwamoto, et al., discloses coating compositions of either anionic silica or cationic alumina combined with cationic resins and water-soluble salts of divalent metals. A gloss-adjusting layer preferably containing colloidal silica is coated or laminated above the ink-receiving layer to obtain a 60-degree gloss of over 10 Gardner units. A single-layer product with acceptable gloss is more desirable. U.S. Pat. No. 6,977,100 to Kondo, et al., discloses coating compositions for an image-receiving layer containing silica, a water-soluble salt of a divalent metal ion, and a polymeric dye-fixing agent comprising cationic moieties, and intended for printing with pigment-based inks. These compositions are not compatible with anionic coating compositions.
It is therefore a primary objective of this invention to enable the manufacture of low-cost, glossy or semi-glossy coated inkjet media that exhibit sufficient porosity and/or permeability such that when said media are printed at high speed using aqueous inkjet inks, the aforementioned defects are reduced or eliminated from the printed images, but which media can be made using anionic coating compositions that are compatible with other anionic coating compositions and which media can be manufactured without risk of adverse interactions with cationic materials in the supply lines during manufacture of the media.